1. Field of the Invention
The present invention relates to a polyester film having a high strength and a high modulus of elasticity and to a process for producing the polyester film.
2. Description of the Related Art
Polyesters represented by polyethylene terephthalate, poly-1,4-butylene terephthalate, poly-2,6-ethylene naphthalate and the like have been widely used in the form of films, fibers and various moldings by virtue of their excellent mechanical properties and heat resistance.
In recent years, a polyester film having a high strength and a high modulus of elasticity has been eagerly desired particularly in the field of films and magnetic tapes.
Means for preparing a high-strength polyester film or tape are known. For example, a method wherein a low-molecular weight compound is added to a polyester resin to prepare a polyester resin composition is known. This method, however, necessitates the steps of extracting the added low- molecular weight compound after the formation of a film and drying the film thus treated. Therefore, this method is unsatisfactory from the viewpoint of productivity. Japanese Patent Publication-A Nos. 66949/1980 and 82150/1980 disclose a method wherein a liquid crystal compound comprising a carbonate or an ester compound is incorporated into a polyester resin composition. In general, however, when these compounds are melt-mixed with a polyester resin, there is a possibility that they will bring about transesterification with the polyester resin to lower the molecular weight of the polyester resin, so that no increase in the strength of the polyester film or tape produced with the use of the polyester resin composition containing a carbonate or an ester compound can be expected.
Furthermore, European Patent Publication-A No. 0491947 (published on Jul. 1, 1992) and the corresponding U.S. Pat. No. 5,162,091 (published on Nov. 10, 1992, Assignee: Kao Corp.) disclose additive compounds for lowering the melt viscosity of polyethylene terephthalate resins and polyester fiber prepared utilizing the resin composition comprising a polyethylene terephthalate resin and the additive compound. Although the melt viscosity of the resin composition is reduced with the use of the additive compound, the strength of the fiber prepared with the resin composition containing the additive compound is the same as that of the fiber prepared with the resin composition containing none of the additive compounds.
In addition, U.S. Pat. Nos. 3,546,164 (published on Dec. 8, 1970) and 4,517,328 (published on May 14, 1985) and Japanese Patent Publication-A No. 76944/1973 (published on Oct. 16, 1973) disclose polyester resin compositions containing a specific additive.